Weldable, ultrahigh tensile steel having an excellent toughness

ABSTRACT

A weldable, ultrahigh tensile steel prepared by adding V and Cu to a low-carbon steel containing Ni and Mo, to which there may be further added W, Nb and Ta, characterized by being able to stand a low temperature and possessing a good weldability and an excellent toughness at a very high level of the strength, particularly the toughness in the welding heat-affected zone, said properties being particularly imparted by the addition of V and Cu.

United States atent Inventors Shozo Seklno;

Toshiyuki Fujishima; Hlroshi Yada, all of Kltakyushu, Japan Appl. No.761,474 Filed Sept. 23, 1968 Patented Nov. 16, 1971 Assignee Yawata Iron& Steel Co., Ltd.

Tokyo, Japan Priority Sept. 27, 1967 Japan 42/621 25 WELDABLE, ULTRAHIGHTENSILE STEEL HAVING AN EXCELLENT TOUGHNESS 10 Claims, 1 Drawing Fig.

[1.5. CI 75/125, 75/123 J, 75/123 K, 75/128 V, 75/128 W, 75/128 G,75/124, 148/31, 148/36 Int. Cl ..C22c 39/50, C22c 39/54 Field of Search75/123,

w mews? Primary Examiner-L. Dewayne Rutledge Assistant Examiner-JosephE. Legru Attorney-Wenderoth, Lind & Ponack ABSTRACT: A weldable,ultrahigh tensile steel prepared by adding V and Cu to a low-carbonsteel containing Ni and Mo, to which there may be further added W, Nband Ta, characterized by being able to stand a low temperature andpossessing a good weldability and an excellent toughness at a very highlevel of the strength, particularly the toughness in the weldingheat-affected zone, said properties being particulariy imparted by theaddition ofV and Cu.

Yleld strength -Oll V- notch Impact value (at- 80C) WELDABLE. ULTRAIIIGHTENSILE STEEL HAVING AN EXCELLENT TOUGHNFSS BACKGROUND OF THE INVENTION1 Field of the Invention The present invention relates to ahigh-toughness, ultrahigh tensile steel having an excellent weldability.

2. Description of the Prior Art l-leretofore, it was known that a steelhaving a yield strength of above 90 lcg/mm. and at the same time arelatively good toughness could be obtained, when adding acarbide-forming element to steel containing 4 to 8.5 percent Ni and thensubjecting the resultant steel to heat treatments such as quenching andtempering, but it was difficult to obtain a steel which could fulfillthe conditions as mentioned in the following, because of the toughnessof the welding heat-affected zone being lowered with an increase in theyield strength, that is, for steels to be placed at the service asmaterials for use in deep sea constructions, particularly such as anouter hull plate of submarine, the following conditions are to beregarded as indispensable:

i. The toughness should be so high as it may stop a further enlargementof cracks of any length occurred at the minimum temperature practicallyapplied and ii. a welding should be easily feasible and the toughness asabove mentioned should be imparted also to the weld.

SUMMARY OF THE INVENTION The main object of the present invention is toprovide a steel which sufficiently fulfills the conditions as abovementioned, that is, a structural steel material which can well stand alow temperature and has an excellent toughness at a very high level ofstrength.

Another object of the present invention is to provide a structural steelmaterial which has an excellent weldability and also an excellenttoughness particularly of the weld.

And still another object of the present invention will become clear fromthe following disclosure, making reference to the accompanying drawing,which shows the relations between the Cu content of the steel of thepresent invention and the yield strength (A), the 2 mm. V-notch Charpyimpact value (B) at 80 C. and the 2 mm. V-notch Charpy impact value (C)of a welding thermal cycle reproducing test piece.

The composition ranges of the steel of the present invention, by whichthe abovementioned objects can be obtained, are as follows:

C 0.04 to 0.15% Si 0.05 to 0.5% Mn 0.05 to 1.5%

Ni 4 to 8.5%

Mo 0.25 to 1.5%

Cu 04 to 3%, preferably 0.5

V 0.04 to 0.I5%

Al an amount usually contained in a killed steel the balance being Feand unavoidable impurities.

Among the abovementioned components Mo may be replaced by a doubleamount of W. Further, to the steel having the composition ranges asabove-mentioned there may be added at least an element selected from thegroup consisting of less than 2 percent Cr, less than 0.10 percent Nband less than 0.10 percent Ta.

As a rule, the steel of the present invention having the composition asabove mentioned is placed at practical services after it has beensubjected to the following heat treatments: Immediately after ahot-working or after the hot-working followed by a normalizing at atemperature of 800 to l,100 C. the steel is quenched from a temperatureranging from 750 to 950 C. and then tempered at a temperature of 500 to650 C. for 0.5 to 3 hours. Depending upon the composition series it isalso possible to omit the quenching.

As above mentioned, in ,a steel containing 4.0 to 8.5 percent Ni, thereoccurs a deterioration of the toughness, particularly in the weldingheat-affected zone in proportion to an increase in the yield strength.But, with the composition ranges of the present invention, which includeespecially 0.4 to 3 percent (preferably 0.5 to 1.5 percent) Cu andfurther 0.04 to 0.15 percent V added together with Cu, the inventors ofthe present'invention succeeded in elevating the strength of thematerial tempered after quenching and at the same time in remarkablyimproving the toughness of the welding heat-affected zone thereof. Therelations between the Cu content and mechanical properties of the steelwill be clearly understood from the attached drawing and the followingdisclosure.

BRIEF DESCRIPTION OF THE DRAWING The drawing involves changes inmechanical properties made in proportion to varying amounts of Cu addedto the steel consisting of 0.1 percent C, 0.25 percent Si, 0.2 percentMn, 6 percent Ni, 0.35 percent M0, 0.1 percent V, 0.03 percent Al andthe balance being Fe and unavoidable impurities, wherein the yieldstrength indicated by a solid line (A) shows a sudden elevation in thevalue from about 0.4 percent on and begins to show a lowering tendencyafter 3 percent, and the 2mm. V-notch impact value at C. indicated by adotted line (B) shows a very high value within the range of Cu, covering0.5 to 1.5 percent and particularly so high as above 18 kg.- mo./cm. at1 percent of Cu, or even within the ranges of 0.4 to 3.0 percent, thevalue above 10 l g.-m./cm., as usually required, which designates anexcellent toughness of the steel of the present invention. Further, alsothe 2 mm. V-notch Charpy impact value at 80 C. indicated by a chain line(C) for test pieces subjected to a welding thermal cycle (with a heatinput of 18,000 Jule/cm.) reproducing test shows an increasing value inproportion to an increasing addition of Cu. It is, however, to note thatin the case of adding no V neither such high strength nor high toughnessof the weld as above mentioned can be obtained, which is clearly seenfrom a reference steel No. E in the tables 3 and 4 to be shown laterindicating the strength and toughness of a steel of the same compositionseries as that of the steel shown in the drawing, but without theaddition of V.

Thus, in the present invention to a steel 4.0 to 8.5 percent- Ni steelthere are added Cu and V and further M0 or W which may be added toreplace a part or a whole of M0. The steel of the present invention thusobtained by adding the elements as above mentioned is characterized bydisplaying an excellent toughness at a yield stress of more than kg./mm.due to a secondary hardening by means of the addition of V and Mo or Win a material a welding heat-affected zone by a mutual function of Cuand V.

In the steel of the present invention C is necessary for a strengtheningof the steel due to the formation of carbide and for maintaining theproper hardenability. In an amount of iess than 0.04 percent the efiectof C cannot be displayed, but in an amount of more than 0.15 percent areduction in the toughness in a welding heat-affected zone isunavoidable.

Therefore, the C content is specified to be within the range of 0.04 to0.15 percent.

Si and Mn are necessary as a deoxidizing agent in making the steel ofthe present invention. However, Si and Mn an amount of less than 0.05percent respectively are insufficient to perform the deoxidizing.However, with an amount of more than 0.5 percent Si the toughness islowered. With an increasing amount of Mn the strength is elevated tosome degree and the toughness in a welding heat-affected zone is alittle improved, but if above 1.5 percent, there appears remarkably atendency of deteriorating the toughness of the material tempered afterquenching. On the basis of the facts above-mentioned Si is specified to0.05 to 0.5 percent and Mn 0.05 to 1.5

percent.

Ni is an element necessary to impart a good toughness to a steelmaterial at the high level of strength. In the steel of the presentinvention an addition of more than 4 percent Ni is necessary in order tokeep the toughness which is more than 90 kg./mm. and is to fulfill theconditions as above mentioned. But, with an addition of more than 8.5percent a specially remarkable improvement in the toughness can not beexpected. Therefore. the addition of Ni is limited to 4 to 8.5 percent.

Mo and V are a carbide-forming element respectively which has a functionof effecting a remarkable secondary hardening by a tempering treatmentwithin a temperature range of 500- to 650 C. and further V has thefunction of improving the toughness in the welding heat-affected zoneunder the coexistence with Cu. In the steel of the present invention asufficient strength can not be obtained by the addition of M in anamount of less than 0.25 percent and V in an amount of less than 0.04percent. On the other hand, if the addition of Mo and V exceeds 1.5percent and 0.15 percent respectively, the effects thereof reach thesaturation and there appear rather tendencies of the steel beingsoftened and the weldability being deteriorated. Therefore, M0 isdefined to be in the range of 0.25 to 1.5 percent and V'in the range of0.04 to 0.15 percent. Further, a part or whole of Mo may be replaced byW.

Cu shows a remarkable effect of preventing the embrittlement of thewelding heat-affected zone, particularly by the mutual function with Vin the Ni-steel, the strength of which has been increased by carbides ofMo and V as above mentioned. in the steel of the present invention anaddition of at least 0.4 percent Cu is absolutely necessary. That is,with an increasing amount of Cu above 0.4 percent its effect ofpreventing the embrittlement of the welding heat-affected zone becomesremarkable, simultaneously accompanied with the effect of increasing thestrength of the steel to some degree. However, if the amount of Cuexceeds 1.0 percent, there appears a tendency of lowering the toughnessof the material tempered after quenching, and particularly, above 3percent, the said tendency becomes notable. Therefore, the amount of Cuis specified to the range of 0.4 to 3.0 percent, preferably 0.5 to L5percent.

Cr has an effect as carbide-forming element of elevating the steel bycausing fine carbonitride to precipitate mainly during a hot-workingandof refining austenitic crystal grains thereby. However, if theaddition of Nb+Ta exceeds 0. l0 percent, the toughness is ratherdeteriorated because of large precipitates of carbonitride being formedin grain boundaries. Thus, the addition of N b+Ta is limited to lessthan 0.10 percent.

W has substantially the same effect as M0 in forming carbide. In thesteel of the present invention a part or whole of Mo may be replaced bya double amount of W without producing any difference in characteristicsof the steel.

Al has an effect of refining crystal grains even with an addition ofsuch a small amount, as is to be added to the steel as a deoxidizer.However, in the steel of the present invention the addition of Al may bedispensed with. However, if it is to be added, it may be added in anamount ofless than 0.07 percent, that is, an amount usually contained ina killed steel.

The steel of the present invention shall be explained more in detail onthe basis of examples.

Alloy steel samples Nos. 1 to 15 having the chemical compositions asshown in table 1 within the ranges specified by the present inventionwere prepared by melting and then subjected to a slabbing and ahot-rolling to make steel plates having a thickness of 13 mm.respectively therefrom. Each steel plate was then normalized at atemperature of 850 C. and thereupon was quenched in water after heatedto a temperature of 800to 900 C. The quenching was followed by atempering at a temperature of 600 C. or 625 C. The mechanical propertiesof each steel plate thus quenched and tempered were shown in table 2. Asthe results of various investigations on said samples to measuresmall-sized 2 mm. V-notch impact values on conditions as shown in table2, (l) and (2) it has been confirmed that in the case of l) the impactvalue is satisfactory, if it exceeds 8.0 kg.-m./cm. at a temperaturelower by more than 20 C. than the minimum temperature practicallyapplied, and in the case of 2) the impact value is satisfactory if itexceeds 4.0 kg.-m./cm. at a temperature lower by 20 C. than the minimumtemperature practically applied to test pieces subjected to a weldingthermal cycle reproduction test.

TABLE 1 Sample N0. of the Composition (percent) steel of the presentinvention C Si Mn N1 M0 V Cu Sol. Al Others 0. 27 0.22 5. 94 0.81 0.110.42 0. 041 0.11 0.18 6.02 0.31 0.15 0.43 0.051 0.17 0.05 6.03 0.35 0.090.48 0.020 Cr, 1.01 0.22 1.36 5.41 0.50 0.10 1.44 0.034 Cr, 1.04 0. 240.47 4.29 0.54 0.10 0. 07 0.012 0.25 0. 22 8.13 0. 23 0.11 0.43 0.0480.27 0.22 6.11 0.40 0.11 0. 42 0.042 0.28 0.24 4. 88 0.33 0.05 1.540.013 0. 25 0.10 5.05 0.35 0.10 2.86 0.002 0.42 0.18 6.02 0.36 0.15 0.48 0.057 Cr, 1.08 0 20 0.25 4.80 0.35 0.10 0.51 0.036 gr, 1.45 r, 1.450.15 1.30 4. as 0.51 0.05 0. 51 0. 044 +1.8 M3 0. 27 0.22 6.17 0.30 0.110. 42 0.042 \1', 0.48 0.20 0. 22 6.05 0.11 0. 42 0. 046 W, 1.57 0.260.32 6.05 0.10 0.47 0.037 \V, 2.05

strength of the steel of the present invention. However, as TABLE 2stronger carbide-forming elements such as Mo and V are used V-notchCharpy in the steel of the present invention, the effect of Cr as thecar- 1139 a g z g bide-forming element is not so much expected.Moreover, as piece ubjigli d to a we 11g Cr has a tendency of deterorating the weldablltty, the addt Sample No notch Charm thermal cycletest tion of Cr may be dispensed with. But, also the addition of Cr ofthe steel of t Yifild impact age (Wil.]ll0ll?=ilt l/npug the present 5reng at -8 01 15M 0 u 0 cm. is efi'ectwe in such a case, where a highhardenability 1s invention kgJmm, kgfim/cmfi required when the contentsof such elements as C, Mn and n n 0 Mo, which elevate the hardenabilityof steel, are those near 3 the lower limits of the respective rangeswithin the composi- 1g; tion of the steel of the present invention.However, even in this 108 .3: 8 case, the addition of Cr is to belimited to less than 2.0 per- 18.? 1.} cent. as it has a tendency oflowering the toughness of the 96 :5 material tempered after quenching,because of a huge carbide g-g being formed thereby, if it is added in anamount more than 94 4,3 required. fg-g Nb and Ta are a strongcarbonitride-forming element 122 9 7 2 112 10.1 7. 3

respectively, which has an effect of elevating the toughness of As isshown in table 2, all of the samples show the yield strength of morethan 90 ltg./mm. and fulfill the conditions as above mentwned. Amongthese samples Nos. 1 and 4, which contain a smaller amount of Crespectively and Nos. 1. 4. 5, 7.

many modifications and variations of the present invention are possiblewithin the composition ranges above mentioned. It is therefore. to'beunderstood that within the scope of the composition ranges as abovementioned the invention may widely 14 and 15 which contain a largeramount of M (or W) be practiced otherwise thanasspecially describedrespectively show such very high impact values at -80 C. for What isclaimed is: the test pieces subjected to a thermal cycle reproductiontest 1. A weldable ultrahigh tensile steel having a yield strength asofmore than 7 kg.-m./cm.. implying an excellent toughness of above 90kgjmm? and at the same time an excellent of these samples This is one ofthe most characteristic featoughness a a low mp rature and goodweldablllty compristures of the present invention. ng

At last, in order to evidence that the composition ranges of t0 5 pecent y Weight C the steel of the present invention should absolutely bykept as 0105 i0 P t y ight Si specified by the present invention, thereare shown in table 3 to p cent by W ight Mn some examples of steelshaving composition ranges different 4.0 to 8.5 percent by weight Ni fromthose of the steel of the present invention in that in one 0.25 to 3.0percent by weight Mo of these examples there is lacking in one ofelements essential 0.04 to 0.15 percent by weight V for the steel of thepresent invention and in another of them 0.4 to 3.0 percent by weight Cuthere is added another of these elements too much or too litthe balancebeing Fe and unavoidable impurities. tle, and the results of the sametests as made in table 1 are 2. The steel claimed in claim 1, wherein Alis added in an shown in table 4. 2 amount of less than 0.07 percent byweight.

TABLE 3 Composition (percent) Reference steel No. C S1 M11 Ni Mo V CuSol. Al Others 0.101 0.24 0.06 3. 75 0. 57 0.05 0.4 0.022 B. 0.005 0.242.48 4.65 0.30 0.14 0.5 0.05 C... 0.10 0.25 0.23 4.110 0.33 0.14 0.014(3.1.05 1). 0.10 0.28 0.23 5.22 0.10 0.13 0. 51 0.0311 (r, 1.04 E. 0.100.20 0.25 4.1121 0.34 0.47 0.030 0.0.22

TABLE 4 3. The steel claimed in claim 1, wherein a part of Mo is t h Chreplaced by a double amount of W. imp gt cvaluo ia i. 4. The steelclaimed in claim 1. wherem a whole of Mo 1s test re laced b adoubleamount of W. 1 b' t d t P Y sil l i ng t ile rmigl 5. The steel claimedin cla1m 1. wherein the steel 1s 0 ole test (wit o o notch Charpy heatinput quenched from a temperature ranging from 750 to 950 C R I t Yielldimpact vagzv J01 1/8,00()l and then tempered at a temperature of 500 to650 C 9 Hence 5 mm at u e 6 A weldable steel ultrahigh tensile strengthhaving a yield tlN. 5 8e 0 Pg [mm 2 m [cm kg strength of above 90leg/mm. and at the same time an excel 99 lent tou hness at a lowtemperature and good weldabilily. 103 5.11 2.6 102 11. 7 2.15 comprisingg is 0.04 to 0.15 percent by weight 0.05 to 0.5 percent by weight S10.05 to 1.5 percent by weight Mn No. A contams a smaller amount of N1,No. B a larger 4D to 85 percent by weight Ni amount of Mn and No. Ccontains no Cu. All of these three 025 to 30 percent by weigh Mo samplesshow insufficient impact values of test pieces sub- 04 to 30 percent byweight Cu ected to a welding thermal cycle reproduction test. No. D has0 04 to 0 15 percem by weight v a smaller amount of Mo and No. Econtains no V. and both le'ss than'z 0 percent by weight Cr samples arenot only low in the strength. as e1ther shows the the balance being Feand unavoidable impuriies yield strength of less than 90 kgJmm. but alsolow 1n the v toughness, as are shown with the impact values of testpieces 7 the Steel cialmed m whefem Al is added m subjected to a weldingthermal cycle reproduction test of ammlm Ofless than 007 ParcelIt yweight below 4 kg.'m./cm., which means that the coexistence of Cu 8. Thesteel claimed in claim 6, wherein a part of M0 is with V isindispensable. It is evident from the foregoing that 5 la d by a doubleamount of W. the composmon ranges f f j 5! pwduce a 9. The steel claimedin claim 6. wherein a whole of M0 is steel as that of the present1nvent1on which 15 d1st1ngu1shed at replaced by a double amount the sametime by an excellent toughness and weldability and particularly is fitfor the uses which the present invention has The Steel claimed m claimthe steeol object quenched from a temperature rangmg from 750:to 950 C.

The examples above mentioned do not designate those and tempewdatatemperamre ofsoo to 650 which limit the scope of the presentinvention. Obviously

2. The steel claimed in claim 1, wherein Al is added in an amount ofless than 0.07 percent by weight.
 3. The steel claimed in claim 1,wherein a part of Mo is replaced by a double amount of W.
 4. The steelclaimed in claim 1, wherein a whole of Mo is replaced by a double amountof W.
 5. The steel claimed in claim 1, wherein the steel is quenchedfrom a temperature ranging from 750* C. to 950* C. and then tempered ata temperature of 500* to 650* C.
 6. A weldable steel ultrahigh tensilestrength having a yield strength of above 90 kg./mm.2 and at the sametime an excellent toughness at a low temperature and good weldability,comprising 0.04 to 0.15 percent by weight C 0.05 to 0.5 percent byweight Si 0.05 to 1.5 percent by weight Mn 4.0 to 8.5 percent by weightNi 0.25 to 3.0 percent by weight Mo 0.4 to 3.0 percent by weight Cu 0.04to 0.15 percent by weight V less than 2.0 percent by Weight Cr, thebalance being Fe and unavoidable impurities.
 7. the steel claimed inclaim 6, wherein Al is added in an amount of less than 0.07 percent byweight.
 8. The steel claimed in claim 6, wherein a part of Mo isreplaced by a double amount of W.
 9. The steel claimed in claim 6,wherein a whole of Mo is replaced by a double amount of W.
 10. The steelclaimed in claim 6, wherein the steel is quenched from a temperatureranging from 750*to 950* C. and then tempered at a temperature of 500*to 650* C.